The present invention relates to an objective tens for an optical pick-up used for an optical disc drive, which is capable of recording data to and/or reproducing data from a plurality of types of optical discs having different recording densities and having different thicknesses of cover layers.
There are various types of optical discs on which digital information is recorded at various densities, respectively. For example, a DVD (digital versatile disc) has a recording density higher than that of a CD (compact disc) or a CD-R (CD Recordable) and has a cover layer thinner than that of the CD or CD-R.
When the recording/reproducing operation for the DVD having a higher recording density is performed, a smaller beam spot diameter is required on a data recording layer of the DVD relative to a beam spot used for the CD having a lower recording density. To provide such a small beam spot for the recording/reproducing operation for the DVD, it is required to use an optical system providing an NA (numerical aperture) higher than an NA attained by an optical system designed specifically for the CD.
The diameter of the beam spot decreases as a wavelength of the laser beam decreases. Therefore, in general, a laser beam having a wavelength of about 660 nm, which is lower than a wavelength of about 780 nm typically used for the CD, is used for the DVD.
To support both of the DVD and CD, recently, optical disc drives employing a laser right source capable of emitting laser beams having different wavelengths are widely used.
In the following, the term “optical disc drive” includes a device designed specifically for a recording operation, a device designed specifically for a reproducing operation, and a device capable of performing both of the recording and reproducing operations.
Japanese Patent Provisional Publication No. HEI 9-145994 (hereafter, referred to as a document 1) discloses an objective lens designed to support both of the DVD and CD. By using the objective lens disclosed in the document 1, laser beams for the DVD and CD are suitably converged onto data recording layers of the DVD and CD, respectively.
The objective lens disclosed in the document 1 has a surface which is divided into more than three regions. An outermost region of the surface is designed such that a beam used for the DVD is suitably converged onto the data recording layer of the DVD. A region formed between an innermost region and the outer most region is designed such that a beam used for the CD is suitably converged onto the data recording layer of the CD. According to this structure, the objective lens can be used as a compatible lens supporting two types of optical discs having different recording densities.
In an optical system employing the objective lens of the document 1, a collimated beam is incident on the objective lens in both of the cases of the CD and the DVD. By this structure, the occurrence of an off-axis aberration such as a coma and astigmatism, which would be caused when the objective lens is shifted by a tracking operation in a direction perpendicular to an optical axis of the objective lens, is prevented.
Recently, new technical standards regarding optical discs having further higher recording densities are proposed. One of such new standard optical discs is an HD DVD (High Definition DVD) having a recording density higher than that of the DVD. The HD DVD has a thickness of the cover layer smaller than or equal to that of the DVD. For the HD DVD, a laser beam (a so-called blue laser having a wavelength of, for example, about 405 nm) having a wavelength smaller than that used for the DVD is required.
With the progress of the practical use of the optical discs having further higher recording densities such as an HD DVD, the demand for optical disc drives which can support existing optical discs (e.g., CD and DVD) and the optical discs of the new technical standards (e.g., HD DVD) is increasing. To accomplish such an optical disc drive, an objective lens that is capable of converging incident beam on the data recording layer of each of the plurality of types of optical discs including the CD, DVD and HD DVD is required.
However, as described above, a conventional objective lens (i.e., the objective lend disclosed in the document 1) is not designed for the new standard optical disc. Therefore, if a beam of the blue laser is incident on the objective lens, the objective lens can not form a beam spot suitable for the recording/reproducing operation for the new standard optical disc because in such a case aberrations including a spherical aberration are caused on a data recording layer of the new standard optical disc.